The invention relates to apparatus for detecting visual field defects of the eye. A particularly important application for such apparatus is in detecting and monitoring the effects of glaucoma.
Instruments are in use which display to a patient various patterns of light spots against a dark background and the practitioner is able to determine visual defects by analysis of those positions where spots are actually seen by the patient. Each pattern of spots is illuminated for a very brief period of time.
In another kind of instrument a single spot present continuously may be moved against a darker background. The positions in the predetermined locus of the spot which are observed by the patient are noted. These methods form the basis for all clinical visual field testing (perimeters). An example of such apparatus is that described in British Patent Specification No. 925,006. This instrument is generally known as the Friedman Analyser.
A difficulty not hitherto generally appreciated is that comparative analysis of visual field defects is difficult because the sensitivity of the eye to the stimulus depends upon whether it is in focus or not. Thus, spots which are not in focus are detected less readily than spots which are in focus. Strictly, perimeters are intended to examine the light-detecting ability of the eye rather than the acuity of the eye and it is therefore advisable to distinguish between the two abilities. This has not been done hitherto. One way of distinguishing light sense from defects in acuity due to refractive error is to insert a correcting lens suitable for each particular eye so that the spots are in focus in every case. However, this is generally not convenient.